This invention relates to a digital multiplex transmission apparatus and system. More particularly, the invention relates to a digital multiplex transmission apparatus and system for multiplexing audio/video information of a plurality of programs and service information, modulating a carrier signal by the multiplexed information and transmitting the signal obtained by modulation to a subscriber device via an optical transmission line.
In CATV broadcasting or CS satellite digital broadcasting, etc., audio/video information of a plurality of programs in which the MPEG-2 TS (transport stream) format is used as the logic format and service information (SI) is multiplexed and transmitted, via a network, to a digital broadcast receiving terminal (referred to as a xe2x80x9cdigital set-top boxxe2x80x9d or xe2x80x9cDSTBxe2x80x9d) installed in one""s home. A subscriber uses the DSTB to receive the multiplexed signal and enjoy a desired program. Examples of programs include digital television broadcast programs, digital audio broadcast programs and on-demand service programs. Various tables make up the service information SI necessary for selecting a desired program from a plurality of programs multiplexed by the MPEG-2 format, decoding the program and reproducing it.
FIG. 9 is a correspondence table illustrating the correspondence among the names of various tables constituting the service information SI, packet identifiers PID and table IDs. The tables are as follows:
(1) PAT (Program Association Table);
(2) CAT (Conditional Access Table);
(3) PMT (Program Map Table);
(4) NIT (Network Information Table);
(5) SDT (Service Description Table);
(6) BAT (Bouquet Association Table);
(7) EIT (Event Information Table);
(8) TDT (Time Date Table);
(9) RST (Running Status Table); and
(10) ST (Stuffing Table).
The first four tables PAT, CAT, PMT and NIT are stipulated by ordinance and are referred to as PSI (Program Specific Information). An overview of the functions of these tables is as follows: (1) The PAT, which is transmitted with a PID of 0, specifies the packet identifier (PID) of a TS packet (transport stream packet) for transmitting the table PMT, which describes the program structure of each broadcasting program. (2) The CAT specifies the packet identifier of a TS packet for transmitting information related to subscription broadcasts (namely decryption and decoding permission information, etc., necessary for decoding a scrambled program). (3) The PMT specifies the packet identifier of a TS packet for transmitting various encoded signals (video and audio signals, etc.) constituting a broadcast program. (4) The NIT transmits information that associates information regarding the transmission path, such as the modulation frequency, and the broadcast program. (5) The SDT specifies information relating to programming channels, such as the name of the programming channel and the name of the broadcast provider. (6) The BAT specifies information relating to a bouquet, such as the name of the bouquet (a group of programming channels) and the programming channels included. (7) The EIT specifies information relating to a program, such as the name of the program, the broadcast date and time and a description of the program content. (8) The TDT specifies the present date and time. (9) The RST specifies the present running status of a program. (10) The ST is a table indicating the invalidation of a table.
FIG. 10 is a diagram useful in describing the hierarchical data structure of MPEG-2 TS PSI. The PAT (Program Association Table) indicates the PID of a TS packet for transmitting, for each program number, the table PMT (Program Map Table) that describes the program composition. Transmission is by a packet whose packet identifier PID is 0. In FIG. 10, the identifier PID of the packet that transmits the PMT of program i is Mi, and the identifier PID of the packet that transmits the PMT of program j is Mj. The PMT indicates the identifier PID of a packet for transmitting a video stream or audio stream constituting the program. This indicates that the identifier PID of a packet for transmitting the video stream of program i is Vi and that the identifier PID of a packet for transmitting the audio stream of program i is Ai. Further, it indicates that the identifier PID of a packet for transmitting the video stream of program j is Vj and that the identifier PID of a packet for transmitting the audio stream of program j is Aj.
Thus, with a digital multiplex transmission system using the MPEG-2 TS format in CATV broadcasting or CS satellite digital broadcasting, audio/video information of a plurality of programs and service information SI is transmitted upon being multiplexed. Consequently, when the number of programs multiplexed increases or the transmission bit rate of a program (audio bit stream/video bit stream) increases, a situation can arise in which the total value of the bit rates of multiplexed information exceeds the maximum transmission rate allowed for the transmission path. The conventional practice in such case is to perform multiplexing while discarding some of the entered MPEG-2 transport streams. However, when MPEG-2 transport streams thus multiplexed with a portion thereof discarded are received and decoded by the DSTB, normal audio/video cannot be obtained.
Accordingly, an object of the present invention is to make it possible to generate normal audio/video in a subscriber device (DSTB) even when the total value of multiplexed information transmission rates exceeds the maximum transmission rate allowed for a transmission path.
According to the present invention, the foregoing object is attained by providing a digital multiplex transmission apparatus comprising (1) a transmission rate detector for detecting transmission rates of video information and audio information of a program; (2) a total transmission rate calculation unit for calculating total value of transmission rates of multiplexed video information and audio information of each program; (3) a comparator for comparing a prescribed transmission rate, which is decided by a maximum transmission rate allowed for a transmission path, with the total transmission rate; and (4) a service information transmission unit which, when the total transmission rate is greater than the prescribed transmission rate, is for transmitting the service information to a subscriber device (DSTB) via the transmission path using a second high-frequency signal that is different from a first high-frequency signal that transmits the video and audio information. The subscriber device has service information decoding means for decoding the service information sent using the second high-frequency signal.
In accordance with this arrangement, some or all of the service information is transmitted to the subscriber device via the transmission path using the second high-frequency signal if the total value of the transmission rates of multiplexed information obtained by multiplexing audio/video information of each program and service information SI exceeds the maximum transmission rate allowed for the transmission path. (Such transmission is referred to as xe2x80x9cbranched transmissionxe2x80x9d.) As a result, the total value of the transmission rates of the multiplexed information is reduced by an amount equivalent to the service information sent by branched transmission, whereby the total value falls below the maximum transmission rate allowed for the transmission path. This makes it possible to obtain normal audio and video at the subscriber device (DSTB).
The digital multiplex transmission apparatus further includes a subscriber control unit (DSTB control unit) for sending and receiving control information (billing information, participation information, etc.) to and from the subscriber device (DSTB) using the second high-frequency signal different from the first high-frequency signal that transmits the multiplexed information obtained by multiplexing the video and audio information of each program. The subscriber control unit, therefore, is used as the service information transmission unit. If this arrangement is adopted, the digital multiplex transmission apparatus and DSTB need not be provided with new hardware for sending and receiving the second high-frequency signals.
Since the event information table EIT has the highest transmission rate of the tables in the service information, only this table is transmitted to the subscriber device using the second high-frequency signal. If this expedient is adopted, it will be unnecessary to transmit all of the service information using the second high-frequency signal. This makes it possible to mitigate the influence on the sending and receiving of control information (billing information, participation information, etc.).
The program specific information PSI in the service information is transmitted upon being multiplexed with the audio/video information. If this expedient is adopted, the program specific information PSI that has been decided by ordinance is not sent by special transmission but can be transmitted in a form multiplexed with the program information.
Further, several tables constituting service information are selected in such a manner that a transmission rate necessary for transmitting these tables will exceed the difference between the total transmission rate and the prescribed transmission rate, and the selected tables are transmitted to the subscriber device using the second high-frequency signal. If this arrangement is adopted, service information is transmitted by the second high-frequency signal to a degree commensurate with the amount of excess. As a result, the amount of information sent by the second high-frequency signal can be reduced and the influence on the sending and receiving of control information (billing information, participation information, etc.) can be mitigated.